Spontaneous Chirality Evolved at the Au–Ag Interface in Plasmonic Nanorods

Chiral ligands are considered a required ingredient during the synthesis of dissymmetric plasmonic metal nanocrystals. The mechanism behind the generation of chiral structures involves the formation of high Miller index chiral facets, induced by the adsorption of such chiral ligands. We found, however, that chirality can also evolve spontaneously, without the involvement of any chiral ligands, during the co-deposition of Au and Ag on Au nanorods. When using a specific Au/Ag ratio, phase segregation of the two metals leads to an interface within the obtained AuAg shell, which can be exposed by removing the Ag component via oxidative etching. Although a close-to-racemic mixture of chiral Au nanorods with right- and left-handedness is found in solution, electron tomography analysis evidences left- and right-handed helicities, both at the Au–Ag interface and at the exposed surface of Au NRs after Ag etching. The helicity profile of the NRs indicates dominating inclination angles in the range from 30° to 60°. Single-particle optical characterization also reveals random handedness in the plasmonic response of individual nanorods. We hypothesize that the origin of chirality is related to symmetry breaking during the co-deposition of Au and Ag, through an initial perturbation at a small region on the Au–Ag interface, which eventually leads to chiral segregation throughout the nanocrystal.